Swift += operator inserts extra ARC traffic and breaks uniqueness checks

[saagarjha]

Description
This thread describes the issue: https://twitter.com/cpatrascudev/status/1661340099190161409. Swift chooses to insert an extra retain/release pair in the loop and this causes the in-place append fast path to not get hit because the uniqueness check fails.

Steps to reproduce

var result = ""

/*
for _ in 1...1000000 {
	result += "abc"
}
*/

for _ in 1...1000000 {
	result = result + "abc"
}

Expected behavior
This should append in-place.

Screenshots

Environment

• swift-driver version: 1.75.2 Apple Swift version 5.8 (swiftlang-5.8.0.124.2 clang-1403.0.22.11.100)
  Target: arm64-apple-macosx13.0
• Xcode 14.3
  Build version 14E222b
• macOS 13.0

Additional context
Built with -O.

[tbkka]

CC: @meg-gupta

[AnthonyLatsis]

There seems to be a more fundamental issue optimizing this pattern:

public struct Foo {
  @_optimize(none)
  mutating func doIt() {}
}

public func test(f1: Foo) -> Foo {
  var f = f1

  for _ in 1...1000000 {
    // alloc_stack & dealloc_stack inside the loop
    //var copy = f
    //copy.doIt()
    //f = copy

    // alloc_stack & dealloc_stack once
    f.doIt()
  }

  return f
}

Revision update: Swift 5.9-dev (c1d5118).